Naphthalimide-phenothiazine dyads: effect of conformational flexibility and matching of the energy of the charge-transfer state and the localized triplet excited state on the thermally activated delayed fluorescence

In order to investigate the joint influence of the conformation flexibility and the matching of the energies of the charge-transfer (CT) and the localized triplet excited (3LE) states on the thermally activated delayed fluorescence (TADF) in electron donor–acceptor molecules, a series of compact electron donor–acceptor dyads and a triad were prepared, with naphthalimide (NI) as electron acceptor and phenothiazine (PTZ) as electron donor. The NI and PTZ moieties are either directly connected at the 3-position of NI and the N-position of the PTZ moiety via a C–N single bond, or they are linked through a phenyl group. The tuning of the energy order of the CT and LE states is achieved by oxidation of the PTZ unit into the corresponding sulfoxide, whereas conformation restriction is imposed by introducing ortho-methyl substituents on the phenyl linker, so that the coupling magnitude between the CT and the 3LE states can be controlled. The singlet oxygen quantum yield (ΦΔ) of NI-PTZ is moderate in n-hexane (HEX, ΦΔ = 19%). TADF was observed for the dyads, the biexponential luminescence lifetime are 16.0 ns (99.9%)/14.4 μs (0.1%) for the dyad and 7.2 ns (99.6%)/2.0 μs (0.4%) for the triad. Triplet state was observed in the nanosecond transient absorption spectra with lifetimes in the 4–48 μs range. Computational investigations show that the orthogonal electron donor–acceptor molecular structure is beneficial for TADF. These calculations indicate small energetic difference between the 3LE and 3CT states, which are helpful for interpreting the ns-TA spectra and the origins of TADF in NI-PTZ, which is ultimately due to the small energetic difference between the 3LE and 3CT states. Conversely, NI-PTZ-O, which has a higher CT state and bears a much more stabilized 3LE state, does not show TADF.


General information
All the chemicals used in synthesis are analytically pure and were used as received.
Solvents were dried and distilled prior to use. 1 H and 13 C NMR spectra were recorded on Bruker Avance spectrometers (400/500/600 MHz). 1 H and 13 C chemical shifts are reported in parts per million (ppm) relative to TMS, with the residual solvent peak used as an internal reference. The mass spectra were measured by HRMS (MALDI-TOF, recorded on a Bruker Ultraflextreme mass spectrometer) and HRMS (ESI-TOF, recorded on an Agilent G6224A mass spectrometer). UV-vis absorption spectra were measured on a UV-2550 UV-vis spectrophotometer (Shimadzu Ltd., Japan). Fluorescence spectra were recorded with an FS5 spectrofluorometer (Edinburgh instruments, UK).
Luminescence lifetimes of compounds were recorded with an OB920 luminescence lifetime spectrometer (Edinburgh Instruments, U.K.). All these calculations were performed with Gaussian 09W [1]. Natural transition orbital analysis were performed by the Multiwfn program [2].
The organic layer was dried over anhydrous Na 2 SO 4 and concentrated under reduced pressure to attain the crude product.

Synthesis of 3.
Compound 2 (300.0 mg, 0.690 mmol) and 5-bromo-2-iodo-1,3dimethylbenzene (300 mg, 0.965 mmol) were dissolved in a deaerated mixed solvent containing toluene (7 mL), ethanol (2 mL), and water (1 mL). After the addition of the catalysts Pd(PPh 3 ) 4 (79.6 mg, 0.069 mmol) and potassium carbonate (285.7 mg, 2.067 mmol), the mixture was stirred for 15 min at room temperature. Then, the reaction suspension was heated to 78 °C and this reaction temperature was kept for 1 h. Following that the reaction temperature was further increased to 110 °C and stirring continued for additional 9 h. Then, the reaction mixture was allowed to cool to room temperature, extracted with chloroform, washed with water and brine. The combined organic layer was dried by anhydrous Na 2 SO 4 and concentrated under reduced pressure. The residue was purified by column chromatography (silica gel, DCM/PE 1:4, v:v). Compound 3 was obtained as pale white solid 3 (150 mg, 60.6%). 1 Figure S2. MALDI-HRMS spectrum of NI-PTZ, 25 °C.    Figure S4. 1 Figure Figure S8. MALDI-HRMS spectrum of NI-PTZ 2 , 25 °C.   Figure S12. 13 Figure S14. MALDI-HRMS spectrum of NI-Ph-Br, 25 °C.    Figure S16. 1 Figure S19. 1 Figure S20. MALDI-HRMS spectrum of 3, 25 °C.    Figure S22. 1        Predominant HOMOLUMO transitions for S 1 , T 1 , and T 2 , between brackets the corresponding coefficient; d ∆ vert corresponds to the energy difference between GS and T 1 and T 2 at the FC region.  b Relative energies at the S 1 optimized geometry, i.e., vertical energy difference.

Vibronic models
For the rate calculations, the Franck-Condon factors, ⟨ | ⟩ , are computed by constructing the vibrational wavefunction as a sum of harmonic oscillators for both the initial and final state. Both these states are considered in the same reference frame, making it possible to connect them through translation and rotation of one of the states. This is accomplished by using the Duschinsky rotation effect (DRE), mapping the normal modes of one state onto the modes of the other state [4,5]. Here, a distinction can be made depending on how to approximate the harmonic normal modes from each state [6]. In vertical models, the same geometry is used for both initial and final state modes, while in adiabatic models the respective minima of both states are used for such a purpose. In addition, one could choose to calculate the gradient and/or hessian at only one or at both states. Representing a final state hessian by an initial state hessian will result in major errors, such that the adiabatic and vertical hessian (AH and VH, see exemplarily for ground and first excited state in Scheme S1) models, which Molecule ∆E S, T-GS (eV) a ∆E S-T (eV) ∆E T-GS (S 1, eV) b ∆E S-T (S 1, eV) 1 [6,7].

Activation energies from the computed adiabatic energy difference gaps and reorganization energies
According to [8], the activation energy for the RISC process (G) can be obtained with where for a RISC E if corresponds to -E S-T (see values in Table S3) and corresponds to the reorganization energies between the initial and final states.